1. Field of the Invention
The present invention provides an automated on-line method and apparatus for inspecting wall thickness of blow-molded plastic containers and, more specifically, it provides feedback regarding the thickness measurements such that the particular container being inspected is associated with a specific mold and spindle.
2. Description of the Prior Art
It has long been known that plastic containers such as bottles must be inspected in order to make sure that the wall thickness is adequate for the desired purpose and that the use of excess material is minimized.
In general, it has been known to employ sampling inspection techniques wherein, at a periodic intervals, which might be on the order of once per hour, a container was removed from the conveyance system after the container emerged from the blow-molder and was destructively tested by cutting the same into multiple, horizontal sections which were then weighed with the weight being correlated with the wall thickness.
An alternative inspection method involved measuring the wall thickness of such containers by nondestructively testing sample plastic containers. A suitable system for effecting such testing is the AGR Top Wave Profiler Gauge PG 9800. A suitable laboratory instrument for this latter approach is that sold under the trademark AGR Top Wave Wall Thickness Profiler. One of the problems with such an inspection approach is that it was time-consuming and labor intensive. Also, the long interval between samplings resulted in a delay in process feedback which in turn could result in reduced production efficiencies.
It has also been known to employ high-speed on-line wall thickness monitoring systems for blow-molded plastic containers. These systems provide real-time monitoring of material distributions and rejection of defects. A suitable system for such purpose is that sold under the trademark AGR Top Wave PET Wall System. While these systems represent a substantial improvement in the completeness of sampling by inspecting each container and the timing of same, they did not provide feedback coordinated with the operation of the blow-molding machine.
U.S. Pat. No. 4,304,995 discloses a system for measuring wall thickness of plastic containers employing infrared absorption. The containers are sampled off-line and required the use of rotation and disclosed the use of radiation sources and radiation detectors which were structured to rotate with respect to each other.
U.S. Pat. No. 4,490,612 discloses a method of measuring the thickness of plastic film using relative absorptions of two infrared wavelengths.
U.S. Pat. No. 5,139,406 discloses the use of infrared absorption in measuring the wall thickness of plastic containers. On-line measurement is contemplated, but this system requires insertion of a probe into the container. Such an approach is uneconomical and inefficient in respect of current blow-molder plastic container production speeds.
U.S. Pat. No. 5,591,462 discloses the use of machine vision technology in monitoring certain defects in blow-molded containers. Among the features being monitored by this system are seal surface, base and neck folds and finish gauge inspection.
PCT publication WO 01/65204 discloses a method and apparatus for measuring plastic containers on-line employing infrared absorption. The apparatus was said to be employable on a conveyer or inside the blow-molder. It made use of laterally homogenous material distribution properties and measured though both sides of the container.
In spite of the foregoing prior art disclosures, there remains a very real and substantial need for an improved inspection system for blow molded plastic containers which will provide timely and accurate feedback regarding not only whether a container fell within the wall thickness specifications, but also identity of the molds and associated spindles which produced the container.